Thermionic amplifier system



July 7, 1931. MlNNlUM 7 1,813,067

'THERMIONIC AMPLIFIER SYSTEM Filed March 18, 1925 Patented July 7, 193-1.

1 UNITED STATES PATENT OFFICE BYRON BaMINNIUM, OF CHICAGO, ILLINOIS, ASSIGNOR, BY MESNE ASSIGNMENTS, TO

STORY & CLARK RADIO CORP., OF CHICAGO, ILLINOIS, A CORPORATION OF DELA- WAKE THERMIONIC AMPLIFIER SYSTEM Application filed Marcli 18, 1926. Serial No. 95,721.

This invention relates to radio receiving apparatus, and more particularly to apparatus of this character making use of radio frequency amplification with the agency of vacuum tubes and repeating coils.

In radio frequency amplifiers there occurs, unless means for preventing such occurrence are provided, what is known as feed-back or regeneration, due to the inherent capacity be tween the plate and grid of the vacuum tubes whereby the grid and plate circuits are coupled together in such a manner as to cause oscillation,as in any of the well-known oscillation generators. ,Such self-oscillation of the tube. results in an unstable condition thereof and seriously interferes with'the reception of signals, andoftentimes sets up undesirable noises in the receiver.

It is a. general object of the present invention to provlde a novel and improved means for preventing feed-back or self-oscillation skilled in the art as the description proceeds.

appended claim.

In said drawings:

Figure 1 is a diagram of radio receiving apparatus involving the principles of the invention; and

Figure 2 is a schematic arrangement of the circuit of the amplifier showing the balanced VVheatstone bridge.

Referring first to Figure 1, there is shown a radio circuit or hook-up in which conventional symbols have been used throughout for the various parts, but a single stage of radio frequency and a detector being illustrated, although it is to be understood that any desirable number of radio frequency stages, each identical with the one disclosed, may be employed, and suitable audio-frequency amplifiers be added after the detector if a greater signal volume is required. The antenna 10 is connected to the ground 11 thru a suitable primary coil 12 of the coupler or repeater 13. The secondary coil 14 of this coupler is tuned by means of the variable condenser 15, connected in shunt there to, to resonance with the incoming signal. One end of the tuned circuit, comprising the secondary 14 and the condenser 15 is connected to the grid 16 of the conventional vacuum tube 17, the filament 18 of which is heated so as to give off electrons by means of the filament battery A, the current being controlled by the rheostat 19. The opposite end of the tuned circuit 14, 15, is connected to the negative side of the filament by means of the conductors 20 and 21, thus completing the so-called input or grid circuit of the tube.

The plate 22 of the tube is energized from the plate battery B through the conductor 23 and the choke coil 24, here illustrated as being provided with an iron core, although this is not necessary. This choke functions to keep the radio frequency currents out of the B battery circuit. The plate may be said to have a shunt connected current supply. A plate return to the filament for the radio frequency currents must herefore be provided. Theconductor 25 connects the plate with the primary of the repeat coil 26, which couples the output of the radio frequency amplifier to the tuned circuit of the detector tube 27, as will be understood from the drawings. The primary of the repeat coil 26 is divided into two parts, 28 and 29, between which parts is connected the stopping condenser 30, of large capacity, its function being merely to prevent the flow of the B battery current directly back tothe positive side of the A battery. Its capacity is so large that it offers no appreciable resistance to the'flow of the radio frequency. currents and therefore need not beconsidered as a part of the radio. frequency'circuit, 3 I 1 7 The circuit as described, is-subject t-o-seltoscillation owing to a tendency for 'radio frequency current to flow from the plate? to the grid in the tubeowing to. theiinherent 7 capacity of these two elements. To counteract this flow, that end of the tuned input circuit opposite the grid is connected,-by means of the wire 20 to the point X, which liesbe tween the two sections'28 and 29 of the primary of the repeat coil'26. Thesesection's are divided at such a point that a substantially neutralizing voltage-Will be applied to the the point X commercially, a small "variable balance, for it is dificult -to exactly locate the point X commerically, a small variable condenser may be connected, as shownin dottedjlines a't'300,,between the filament and grid, or-if necessary; if the potentialof the point X is too high,i-between the grid and plate; This condenser can be adjusted to give exact balance or neutralization, asit is sometimes termed.

In Figure2, the variouselements of Figure 1' involved in the radiofrequency amplifier are arranged in'the form of a'Wheatstone bridge. The four points or junctionsof the bridge are let'teredrespectively F, G, P and X "F representing the. filament 18,. Gr the grid'lG, P.;the plate 22 and Xthe-junction'X' of the two parts of'the repeat coil 26. *Be tween G and P,'the'small condenser 31represents the grid-to-pla't'e' capacity "ofith'ej tube;

1 Between the points Fand G, the small'condenser 32 represents the capacity between the filament and grid in the tube; DhQSQ COD densers comprise two'arms of the bridge-and the other two are composed'of the parts 28 a r lf-oscillation; can Will be readily ap Co parent tli'at when-the bridge is balanced by a,

The condenser 300 has been omitted, since, as explained, it forms no partof the radio is claimed as new a'nd desired to Letters Patent is: V Radio frequency receiving apparatus comprising incombination, a vacuum tubehaving versely, any changes in theelectrical con- .ditions of the points G and X,-due to variation in the input circuit of the tube, cannot have any effect on the potentials of the pomts P andF- In this manner the grid and plate can be" consideredas electrically isolated, for 7 the purpose of balance and the tube circuits "are thus inherently balanced and stable.

The tuningcircuitbei ng connected to 0P1 posite points of thebridge does. not, byits variation, affect the balance and the same may besa-idof the B batterycircuit.

Having thus described the invention, what a filament, a grid and a plate, an input inducbe secured by tance, an output inductance-and a source of plate supply, said elements being arranged in the form of a balanced Wheatstone bridge, the grid-to-plate capacity of the tube" being in one arm of the bridge, the filament-to-grid capacity being in another arm, said output 7 inductance being divided into two portions by a stopping condenser, each portion being in its allotted one of the two remaining arms of said bridge, with said stopping condenser in one inductive arm,'anda secondary'circuit coil inductively coupled to both parts-of said output inductance, V

In testimony whereof hereunto affix signature, 1 Y

' BYRON'BLMINNIUM.

an d'29 of the primary ofthe repeat coil 26.:

inhelfin b l n fiofthbfidg and will clear 7 I ly illustrate the means for preventing 1 feedf proper"corelationfof the condensers 32am 31 and thecoil's 28and29, that any electrical disturbances occurring between the conj ugate I unctions and cannothave any effect on the-potentials or the junctions G and XI Gon roe 

